The field of solar generation suffers for a variety of reasons, including: poor energy conversion efficiency, high installation cost, high generation cost per watt hour, limited dispatchability, poor energy storage options, variable demand, high maintenance cost, high capital cost, poor reliability, low manufacturing volume, and poor aesthetics. Dispatchability is compromised by environmental factors such as: seasonal cycles, diurnal cycles, temperature, wind, and cloud cover. Efficiency is limited by airborne dust, dirt, and shadows. Other variables include: site placement, collector type, collector packaging, tracking, angle, latitude, cleanliness, and power conversion systems. Varying degrees of these factors severely limit public acceptance. All solar generation systems benefit from free fuel. Some solar generation systems have the benefits of distributed generation systems: no service interruptions due to transmission lines and very limited distribution losses.
Photovoltaic (PV) based systems suffer efficiency drops due to spectral mismatch, recombination losses, and resistive losses. Spectral mismatch occurs when the wavelengths of solar radiation, also called insolation, do not match the wavelengths of the semiconductor bandgap. Insolation wavelengths of higher energy than the material bandgap may generate an electron—hole pair at the PV bandgap energy with the remainder of the energy converted to heat. Insolation wavelengths longer than the material bandgap are simply converted to heat. Higher temperature and crystal defects increase recombination losses as a function of PV area. To maximize power transfer, the electric impedance of the source, a function of incident radiation, must be matched to the electric impedance of the load. Thus a system operating at maximum efficiency is subject to a collapse in delivered energy if the source impedance rises or the load impedance drops, such as from a motor starting, switch mode power supply, or a load step. Debris, dirt, or bird droppings may shadow a single cell, dropping efficiency by significantly more than the shadowed area. PV is generally not operated at the maximum power point, except when batteries are being charged, and then not at all times of day to save the expense of power electronics to capture the noon sun. More complex PV structures can offer some efficiency improvement, but have the same basic limiting factors. Without storage or backup generation, PV systems are not dispatchable. Solar energy input goes to zero at night. Energy storage is either very costly, or completely unutilized if economics dictate a system that is only capable of providing supplemental levels of energy, drawing the remainder of the energy need from the utility grid. The grid is not an energy storage device. It merely moves the problem of dispatching energy when needed from the end user to the electric utility. Energy storage in the form of lead acid batteries poses significant maintenance and environmental impact. Gridless operation frequently requires a backup generator to keep the size of batteries and PV cells within economic limits, except in very remote areas. Minimum power output requirements of grid independent solar systems during low insolation winter months can dramatically increase the required collector area and therefore cost.
Thermophotovoltaic (TPV) systems take a thermal source, create an optical emission, and photovoltaically generate an electric output. These systems generally suffer a mediocre, but better than solar, spectral match between the emitter and PV, poor efficiency for the capital cost, need for a high grade thermal source, parasitic losses, and low bandgap energy PV. These systems benefit from: better spectral match than pure PV, no partial shadowing, and reliability of a solid state device. Some systems include diurnal energy storage.
Solar thermal systems benefit in efficiency over PV systems in that the entire incident spectra is converted to heat. The heat is frequently used to spin a turbine, to generate electricity. However, these systems are generally too complex for non-utility users to maintain and operate. Diurnal storage and increased dispatchability is incorporated in systems such as trough based systems, such as SEGS, dish systems such as described in U.S. Pat. No. 5,932,029 “Solar Thermophotovoltaic Power Conversion Method and Apparatus”, heliostat field systems such as Solar Two, and solar tower systems such as proposed by Solar Mission Technologies. Utility scale generation generally does not give the reliability of an Uninterruptible Power Supply due to distribution failures. Efficiency is reduced with distribution losses.
Waste heat is rarely recovered. When waste heat is available in batches, a primary source of fuel is consumed to fill in the interruptions. Storage of waste heat is not considered economical.
Thus a need has arisen for a distributed electrical generation system and method to overcome the limitations of existing systems.